Vehicle systems may be equipped with a fuel system including a bifurcated fuel tank for storing and delivering fuel to an internal combustion engine. For example, a fuel tank may include a reserve tank, or the fuel tank may be configured as a saddle tank in order to achieve compact packaging without compromising fuel storage. For example, saddle tanks may be utilized in all wheel drive (AWD) vehicles in which the AWD hardware occupies significant underbody space that is required for packaging a single tank. Further, saddle tanks may be designed to hold more fuel than standard fuel tanks, making them more desirable in vehicle systems that require more fuel storage, such as AWD vehicles.
Bifurcated fuel tanks typically include two compartments, a first, “active” compartment and a second “passive” compartment, that are fluidly coupled. A fuel pump may be coupled to the active compartment, which is maintained at a full capacity by a jet pump that draws fuel from the second compartment to replenish the fuel withdrawn by the fuel pump.
Bifurcated fuel tanks may include a dedicated fuel level indicator, such as a floating sensor, within each of the first and the second compartments, to determine an amount of fuel remaining in the respective compartment. An overall fuel level indicator, such as an in-dash fuel level indicator, may indicate a total amount of fuel to the vehicle operator, which may indicate an average of the outputs of the fuel level indicators in each compartment.
Over time, one or more of the fuel level indicators may become stuck, malfunction, or become decoupled from the vehicle powertrain control module. As a result, the fuel level in one or more compartments may become unknown or not reflective of the actual fuel level within the compartment. As such, the overall fuel level indicator may provide an inaccurate or indeterminate fuel level to the vehicle operator. One common failure associated with fuel level indicators occurs when the fuel tank experiences excessive vacuum levels due to leaking purge valves. The excess vacuum in the fuel tank can cause an arm of the floating sensor to bend. A bent arm results in the sensor over-estimating the amount of fuel in the tank. This may lead to the vehicle running out of fuel during a trip.
Periodically, diagnostic tests may be performed on the fuel level indicators included within the compartments of a saddle tank. One example approach for diagnosing fuel level indicators in saddle tanks is shown by Sweppy et al. in US 2014/0260576. Therein, engine vacuum or a vacuum pump is utilized to reduce an initial fuel tank pressure to a reference pressure, and a time taken to reach the reference pressure is utilized to determine a rate of pressure change. A fuel fill level is then inferred based on the rate of pressure change. For example, a high level of fuel in the tank is inferred when the rate of pressure change is high. The inferred fuel level is then compared to a status (e.g., high, low, etc.) of the fuel level indicator.
However, the inventors herein have identified potential issues with such an approach. As an example, any leak in the fuel system may alter the rate of pressure change. Therefore, the rate of pressure change responsive to an applied vacuum does not always correlate with actual fuel levels. This leads to inaccurate diagnosis of the fuel level indicators. For example, fuel system leaks may decrease the rate of pressure change. As a result, a lower fuel level than the actual fuel level may be inferred. Under such conditions, since the indicator status does not correlate with the inferred fuel level, the fuel level indicator is diagnosed to be degraded even if it indicates the actual fuel level. Further, in order to obtain diagnosis of the entire range of fuel level indicator output, Sweppy's method may require monitoring output of the fuel level indicator over a duration (e.g., period of 100 miles) of engine combustion. Consequently, the diagnosis may take a long time to complete, particularly in hybrid-electric vehicles, or other vehicles configured to operate for extended periods with little or no fuel expenditure. Additionally, fuel sloshing may occur during sharp vehicle maneuvers, resulting in fuel transfer between compartments, and generating a fuel vapor pressure spike. During an evaporative emissions leak test, fuel sloshing may result in an incorrect diagnosis. This is often mitigated by aborting such tests responsive to a deviation in indicated fuel level. However, in saddle tanks, the overall fuel level does not change during a fuel sloshing event. Thus the accuracy of both fuel level indicators is needed to ensure the robustness of evaporative emissions leak tests.
In one example, the above issues may be addressed by a method for an engine, comprising: indicating degradation of one or more of a first fuel level indicator coupled to a first compartment of a fuel tank and a second fuel level indicator coupled to a second compartment of the fuel tank based on a deviation of a fuel tank pressure from a steady state pressure during a refueling event; and indicating a fuel tank level based on the fuel tank pressure responsive to the degradation. By diagnosing fuel level indicators during refueling, noise factors, such as noise due to fuel sloshing, may be reduced. Further, due to the high flow rate of fuel dispensed into the tanks, the pressure changes that occur during refueling are large. Consequently, the pressure changes are robust to other fuel system deficiencies such as a fuel system leak, for example. Hence, faster and more accurate fuel level indicator diagnosis may be obtained.
As an example, a timing of a first fuel tank pressure spike following a first steady state pressure during a refueling event may be utilized to determine when the first compartment reaches full capacity. The first pressure spike is indicated by a fuel tank pressure sensor due to the fuel spilling over to the second compartment upon the first compartment reaching full capacity. If the first fuel level indicator output indicating that the first compartment has reached full capacity is asynchronous from the first pressure spike, degradation of the first fuel level indicator is indicated. For example, the first fuel level indicator may indicate full capacity prior to the pressure spike when the fuel indicator has a bent float arm.
Further, a deviation from a second steady state pressure may be utilized to determine when the second compartment reaches full capacity. Following the first pressure spike, as the fuel is being transferred from the first compartment to the second compartment the fuel tank is maintained at a second steady state pressure. Upon the second compartment reaching full capacity, a deviation from the second steady state pressure is indicated by fuel tank pressure sensor. If the second fuel level indicator output indicating that the second compartment has reached full capacity is asynchronous from the deviation from the second steady state pressure indicated by the pressure sensor, degradation of the second fuel level indicator is inferred. For example, the second fuel level indicator may indicate full capacity prior to the change in the second steady state pressure when the fuel indicator has a bent float arm.
In this way, by correlating the timing of the fuel level indicator outputs reaching full capacities with the deviations from the steady state pressures occurring during refueling of saddle tanks, both fuel indicators may be diagnosed in a single refueling event. As a result, faster diagnostics may be achieved. Further, during refueling, due to the high flow rate of the fuel entering the tank, relatively large pressure changes (such as the pressure spike when the first compartment reaches full capacity and the deviation from the second steady state pressure when the second compartment reaches full capacity) are generated. As a result, the pressure changes can be detected with high accuracy even if there are leaks present in the fuel system. Taken together, by diagnosing one or more fuel level indictors in a saddle tank based on changes in fuel tank pressure during a refueling event, the technical effect of a faster, more complete, and accurate diagnosis may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.